Boston Celtics guard Jason Terry reacts during Sunday’s playoff victory against the New York Knicks in Boston.

BOSTON — It started with a 3-pointer by Avery Bradley 16 seconds into the game. It ended with a layup by Jason Terry with 6.5 seconds left.

Yes, the Boston Celtics finally got their offense going.

“It’s coming,” Terry said. “You can feel it.”

It was there on Sunday — for much of the game, anyway — when the Celtics beat the New York Knicks 97-90 in overtime to avoid being swept in the opening round of the playoffs.

But if it’s not there on Wednesday night in New York, the Celtics will have a very tough time forcing a sixth game Friday night in Boston.

The Celtics scored fewer than 20 points in six of the first 10 quarters in the series. They managed a meager eight points in the fourth quarter of the opener — one less than Terry scored by himself in the last 1:32 of overtime on Sunday. They scored 78, 71 and 76 points in the first three games after scoring fewer than 80 just five times during the regular season.

“We’ve got a lot of basketball in us,” Terry said, “but there’s always something like one quarter that holds us back. So if we can put together four quarters of great Celtics basketball, ball movement, getting out in transition, then this series is going to be a long one.”

The Celtics made 51.3 percent of their shots in the first half Sunday after averaging 39.5 during the first three games.

But they dropped to 25 percent (4 for 16) in the third quarter when the Knicks cut a 59-39 deficit to 68-65 entering the fourth.

And they finished with just three offensive rebounds — and only two second-chance points — in 53 minutes.

The Knicks got five offensive rebounds from Tyson Chandler and four from Iman Shumpert.

But Boston managed to win when New York’s own shooting woes continued in overtime with only two field goals in eight attempts. Carmelo Anthony hit just one of four shots in overtime and finished at 10 for 25, despite leading all scorers with 36 points.

“We didn’t shoot the ball well,” Anthony said, “and we still put ourselves in a position to win the basketball game. There’s an upside to that.”

The downside for him was the Celtics’ defense, particularly that of Brandon Bass. Before fouling out with 4:27 left in the fourth quarter, he guarded Anthony tightly, contested shots and kept the NBA’s leading playoff scorer from getting hot.

Anthony missed all seven of his 3-pointers and got 16 of his 36 points on free throws.

Boston coach Doc Rivers praised Bass’ performance.

“He was the star of the game,” Rivers said. “He just defended and did it over and over and over again.”

The Knicks got back in the game after Anthony picked up his fourth foul with New York trailing 65-51 with 3:35 left in the third quarter. With Anthony on the bench, Raymond Felton scored 11 points in a 14-3 run that cut the deficit to 68-65.

“The fact that we came back from a 20-point deficit in that building shows a lot about our team,” Felton said.

When they get to their own building, they’ll have their second leading scorer back. J.R. Smith, the NBA’s Sixth Man of the Year, was suspended for Sunday’s game for hitting Terry with an elbow in New York’s 90-76 win in Game 3. Smith is averaging 16.3 points per game in the series.

Felton helped take up the slack with 27 points, but Smith’s perimeter shooting will put more pressure on the Celtics’ defense and, perhaps, open up the lane for the Knicks’ offense.

“You just don’t know what the impact was” of Smith’s absence, Rivers said, “but guarding one less guy can’t hurt.”

Smith’s return may be too much for the Celtics, especially after their best players put in a very long day with 35-year-old Paul Pierce playing 49 minutes, 51 seconds and 36-year-old Kevin Garnett going 36:57. They’re hoping that two days between games will be enough time for them to recover.

But the Celtics need a lot more than their veteran stars. Terry proved that on Sunday.

“He’s got to be our X-factor,” Pierce said. “Guys are going to load up to me, load up to Kevin, and he’s got to be our X-factor and we depend on him for that.”

“There’s just so much pride when you look around,” Pierce said, “the banners, the crowd and everything going on with the Boston Celtics history.”

That will be missing in Madison Square Garden on Wednesday night.

Just one more victory and the Knicks will celebrate their first playoff series win since 2000, when they reached the Eastern Conference finals. And no NBA team has ever won a series after losing the first three games.

“We played all year to get homecourt advantage and if you get one (win) on the road, that’s a major plus,” Knicks coach Mike Woodson said. “We just have to go home and handle our business.”